Triplet-Sensitized Cis-Trans Isomerization of the Protonated Schiff Base of Retinal Isomers

Pulsed (N₂ laser) and continuous excitation methods are applied to ethanol solutions of the protonated Schiff bases of retinal isomers (PRSB). PRSB's triplet states, produced via energy transfer from the triplet state of phenanthrene, are found to undergo cis-trans isomerization processes. The observed yields of the triplet-sensitized isomerization (ϕ^T iso) for the 9-cis, 13-cis, 11-cis, and all-trans isomers are, respectively, 0.5 ± 0.2, 0.2 ±0.1, 1.0 ± 0.2, and <0.05. The large yield for the 11- cis isomer is rationalized in terms of the destabilization of the first excited triplet of this isomer due to torsion around its 12- 13 single bond. Calculations indicate that this leads to a substantial decrease in the barrier for twisting around the double bond and thus may be responsible for the exceptionally high ϕ^T iso value observed. The relevance of this analysis to the photochemistry of visual pigments is discussed.